1. Field of Invention
The present invention relates to a method for fabricating a read-only memory (ROM). More particularly, the present invention relates to a method for fabricating a mask read-only memory (Mask ROM).
2. Description of Related Art
Read-only memory (ROM) is non-volatile and can retain data as disconnected from power supply. Therefore, various electronic products use ROM to store data needed for their operations. In the family of ROM, the simplest one is namely the Mask ROM, which uses MOS transistors as memory cells and is programmed by implanting ions into the channels of selected memory cells to change the threshold voltages thereof. The presence of implanted ions determines the conducting state (On/Off) of a memory cell during reading operation.
A Mask ROM comprises a plurality of buried bit lines in a substrate and a plurality of polysilicon word lines on the substrate crossing over the buried bit lines, while the substrate under the word lines and between the buried bit lines serve as the channels of the memory cells. The process of implanting ions into the channels of selected memory cells is called coding implantation, and the data (0 or 1) stored in each memory cell is dependent on the presence of implanted ions.
Refer to FIGS. 1xcx9c2, wherein FIG. 1 illustrates a top view of a Mask ROM in the prior art, and FIG. 2 illustrates a cross-sectional view of the Mask ROM along line Ixe2x80x94I. The Mask ROM comprises a substrate 100, parallel buried bit lines 104 and parallel word lines 102 crossing over the buried bit lines 104, wherein the word lines 102 are isolated from the substrate 100 by a gate oxide layer 108. The Mask ROM is programmed by implanting ions into the channels of selected memory cells, such as the channel of the memory cell under a coding window 106, to change the threshold voltages thereof. The presence of implanted ions determines the conducting state (On/Off) of a memory cell during reading operation.
Refer to FIG. 2 again, a patterned photoresist layer 110 is formed with a coding window 106 by using a lithography process. A coding implantation 112 is performed to implant ions into the substrate 100 under the word line 102 exposed by the coding window 106, so as to program the Mask ROM.
As the Mask ROM cell is miniaturized to increase the integration of the Mask ROM device, a photo-mask with small aperture sizes is required to form small coding windows, otherwise coding errors may occur because of the misalignment between larger coding windows and selected channel regions. However, fabricating a photo-mask with small aperture sizes is time-consuming, and is more difficult and more expensive.
Moreover, since the corners of the coding windows are rounded because of light scattering, coding errors easily occur in the prior art.
Accordingly, this invention provides a method for fabricating a Mask ROM with self-aligned coding without using a photo-mask with small aperture sizes, so as to reduce the fabrication cost of the photo-mask.
This invention also provides a method for fabricating a Mask ROM to prevent coding errors caused by deformation (corner rounding) of the coding windows, so as to improve the programming accuracy.
A method for fabricating a Mask ROM with self-aligned coding of this invention comprises the following steps. A plurality of buried bit lines are formed in a substrate, and then a conductive layer and a first blocking layer are sequentially formed on the substrate. The first blocking layer and the conductive layer are patterned into a plurality of first blocking strips and a plurality of word lines, respectively, and then a plurality of second blocking strips are formed between the word lines and between the first blocking strips. Thereafter, a first photoresist pattern is formed over the substrate, and then the first blocking strips are patterned into an array of blocking bumps, which define a plurality of pre-coding windows with the second blocking strips. After the first photoresist pattern is removed, a coding photoresist layer is formed on the substrate with a plurality of coding windows exposing selected pre-coding windows, wherein a coding widow is larger than a pre-coding window. Then, a coding implantation is performed using the coding photoresist layer, the blocking bumps and the second blocking strips as a mask to form implanted coding regions in the substrate under the pre-coding windows exposed by the coding windows. The coding photoresist layer is then removed.
Since the blocking bumps and the second blocking strips together define the pre-coding windows and block the regions outside the coding regions, the coding implantation can self-align to the coding regions under the coding windows to avoid coding errors even if misalignment occurs. Therefore, the coding windows can be formed larger without causing coding errors.
Moreover, each coding window is surrounded by two blocking bumps and two of the second blocking strips, and therefore has a tetragonal shape. Since the coding implantation profile of a selected memory cell under a coding window is determined by the shape of the corresponding pre-coding window, corner rounding of the coding windows will not adversely affect the coding implantation profile to cause coding errors.
Furthermore, since the coding windows can be formed larger, the aperture sizes of the photo-mask can be increased to reduce the cost of fabricating the photo-mask and to shorten the turn-around time (TAT) of mass production.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.